One of Taiwan's last diplomatic allies has cut ties with the island in favour of Beijing, just days after a new president was voted in in Taipei.
Nauru, a tiny Micronesian island, was just one of 12 countries that kept diplomatic ties with Taipei.
But in recent years, Beijing - which insists Taiwan is part of China - has been poaching its diplomatic allies.
Taiwan suggests this latest loss is related to the weekend's election results, which angered China.
The election saw voters pick pro-sovereignty candidate William Lai as their next president, a man Beijing has labelled a "troublemaker" over remarks he made in the past supporting Taiwanese independence, which it sees as a red line.
"This timing is not only China's retaliation against our democratic elections but also a direct challenge to the international order," Taiwanese officials said after Nauru's government announced it would "no longer recognise [Taiwan] as a separate country but rather as an inalienable part of China's territory".
In a media conference on Monday, Taipei's deputy foreign minister Tien Chung-kwang accused China of taking advantage of recent "political fluctuations" in Nauru to "buy over" the country with financial aid.
"China thinks it can suppress Taiwan with such methods, I think it is wrong. The world has noticed Taiwan's democratic development. If [Beijing] continues to use such despicable methods to seize Taiwan's diplomatic relations, democratic countries all over the world will not recognise it," Mr Tien said.
Still, his ministry remains "on strong alert" to combat further moves from China to isolate Taiwan on an international stage, he said.
China - which sees the self-ruled island of 23 million as a breakaway province that will eventually be under Beijing's control - welcomed Nauru's decision.
"The decision of the government of Nauru to resume diplomatic relations with China fully demonstrates once again that the one-China principle is the will of the people and the trend of the times," China's foreign ministry said.
This is not the first time Nauru has cut ties with Taiwan, which considers itself distinct from the Chinese mainland, with its own constitution and democratically elected leaders.
In 2002, Nauru made a similar diplomatic switch to China - it later restored relations with Taiwan in May 2005.